Typical fuel injection systems include at least one piezo injector, e.g., a piezo diesel injector. Such a piezo injector may include a piezo actuator which activates a closure element of the injector in an indirect way, e.g. via a servo valve. In such an injector with a servo valve, the injected quantity of fuel is defined essentially by the opening period of the servo valve. In this context, the opening time of the servo valve is measured on the basis of an electric feedback signal of the piezo actuator which acts as a sensor. However, with this method it is not possible to measure the closing time of the servo valve, since in this context the amplitude of the force acting on the piezo actuator is too low. As result, in this context there is still uncertainty with respect to the opening period of the servo valve.
It is known to estimate the closing time of the servo valve on the basis of three items of information:                the servo valve opening time,        the electrical energy applied to the piezo actuator, since this energy correlates with the change in shape of the piezo actuator, and        the discharge time.        
However, this estimation method is influenced by various interference factors and is relatively inaccurate. For example, accuracy of +/−5 μs is not typically achieved (a typical value of the required accuracy of the injection quantity).